It seemed to me as a historian that the concept of extremism begged a question: how do certain ideas, movements and political impulses come to be considered extremist? As a citizen whose political identity was shaped by the late twentieth century, I saw the militias’ assertion of a right to use armed force to change government policy as new, threatening, and beyond the pale of legitimate politics. But as a historian of early America I found achingly familiar their assertion of a right to take up arms to prevent the exercise of unconstitutional power by the federal government. As a historian, then, I was faced with a more specific question: how has the United States as a political society come to view the assertion of that right as extremist?